The invention relates to an apparatus including a catalytic converter with electric heating means for activating the catalyst of the catalytic converter.
Catalysts accelerate chemical reactions or control chemical reactions so that they occur in certain ways (selectivity). The chemical efficiency and also the conversion process for generating a certain desirable product is increased. Basically, a catalyst only affects the kinematics of a particular reaction. The thermodynamic conditions are not changed.
A distinction is made between homogeneous and heterogeneous catalysts. In the last mentioned type, catalysts and reactions are in different phases. These catalysts which are often called xe2x80x9ccontactsxe2x80x9d are used mainly for the acceleration of gas phase reactions such as oxidation hydration or steam reformation.
With heterogeneous catalysts, the reactants are adsorbed on the surface of the catalyst where they are converted to species, which are capable of reacting. The reaction occurs by way of diffusion effects and the products are then desorbed.
Independent from the reaction system under consideration energy is required for the conversion of the reactants to species capable of reacting. This kind of energy will be called below the activation energy of the catalyst. For endothermic reactions (for example, steam reformation) the activation energy must be supplied permanently from the outside already for thermodynamic reasons. In exothermic processes (for example, catalytic oxidation), the energy needs to be supplied from the outside only to get the reaction started. Subsequently, the energy released during the reaction covers the activation energy of the catalyst.
For an effective acceleration of a reaction, a catalyst with sufficient conversion capability (activity) as well as a minimum amount of energy must be available.
In industrial applications of catalysts, sufficient energy is available from optimized reactor- and burner systems or by preheating of the reactants. For small-scale applications, those solutions however are too expensive.
An example of a small-scale application is an exhaust gas catalytic converter of a gasoline engine. In this case, the exhaust gases generally provide sufficient energy for a rapid elimination of pollutants. However, during cold startup or idling operations, three-way converters of automobiles do not operate very well, since under these circumstances, the exhaust gas does not supply sufficient thermal energy.
DE 41 10 395 A1 discloses an arrangement for solving this problem. It includes an electric heater with honeycomb-like channels, which extend in parallel. A start-up catalyst is disposed in the honeycomb-like channels in the form of layers. The catalytic converter can be heated by a heater as disclosed in the printed publication DE 41 10 395 A1. The heating energy required by the start-up catalyst can be rapidly supplied.
It is the object of the present invention to provide an arrangement including a heterogeneous catalyst which is suitable for small-scale use and which has an improved efficiency.
In an electrically heated catalytic converter including a heater consisting of a structure having an open porosity so as to permit fluid to flow therethrough, a heterogeneous catalyst is disposed within the heater structure. The heater structure, which includes the catalyst, is capable of accommodating sufficient electrical energy to provide the activation energy required for a particular catalytic reaction to be performed by the catalytic converter.
The pores are arranged irregularly so that gas passing through the pores is repeatedly deflected. The expressions xe2x80x9copen porosityxe2x80x9d as used herein does not cover parallel honeycomb-like channels as they are known from DE 41 10 395 A1. Furthermore, the heterogeneous catalyst is arranged in the interior of the heater body.
The required activation energy of the catalyst is supplied to the reaction by way of the electric heater. Electric energy is converted to heat energy and provided as activation energy for the catalyst. Since a direct coupling with the catalyst is provided, the energy can be applied directly at the location where it is needed for the catalytically supported reaction. The energy density can be controlled steplessly by controlling the electric power supplied to the heater so as to achieve high efficiencies. The required thermal energy is generated in situ. Energy losses occur essentially only as a result of the heat capacity of the materials involved. For this reason, also good energy efficiencies can be achieved.
The porosity of the heater material provides for a particularly large surface. The advantageous results achieved by such a large surface area will be pointed out below on the basis of exemplary embodiments.
As an electric heater, which is particularly suitable for the conversion of electric energy to heat energy, an electric semi-conductor body is used. In the semiconductor body, electric energy is converted to heat energy with high efficiency.
In a very simple embodiment of the invention, the catalyst is arranged loosely within a container-like electric heater consisting of a semi-conductor material. With this arrangement, problems of attaching the catalyst to the electric heater are avoided.
As electric heater for example, the porous flow-through heating element known from DE 41 30 630 may be provided. If this heating element is tubular, the catalyst material may be placed into the tubes as a loose fill. The current is supplied to the heating element in a way as it is known for example from DE 42 09 685. Fluid extracts are supplied to the catalyst preferably through the tube wall. In this way, not only the catalyst, but also the extracts are heated in a simple manner.
In another embodiment of the invention, the semi-conductor is identical with the catalyst material. The semi-conductive material then has a double function. The catalyst is then activated in an optimal way and there are no mounting problems. Doped materials of the group of the perovskites can be used. However, such cases are considered to be an exception as they are normally expensive.
In another embodiment of the invention, the catalytically active material is attached to a support structure, that is, to a carrier having an open porosity. In this way, the catalyst is heated directly and over a large area. In this way, the activation energy for the catalyst is supplied especially fast and economically. In addition, the catalyst then has the well-known large surface area to achieve large-scale chemical conversions. This embodiment can be realized in most cases. It finds more widespread applications than the two previous embodiments. A disadvantage is however that the catalyst material can detach from the support structure and get lost.
Upon selection of an electrical semi-conductor material for the electric heater, particular attention must be given that the material can withstand thermal stresses and is corrosion resistant in order to obtain a long operating life. Ceramic semi-conductor materials such as SiC doped with boron or nitrogen are particularly suitable. Also to be considered are transition metal carbides such as titanium carbide, tungsten carbide and nitrides such as titanium nitride.
The catalyst material is applied to the semi-conductor material by a coating or a combination method. The selection of the catalyst depends on the respective field of application for the system. Basically, the invention can be used with any known catalyst.
In another embodiment of the invention, the catalyst material is cemented to a semi-conductor material. A ceramic cement is suitable for this purpose. This kind of attachment is simple and reliable.
Particularly suitable are catalyst materials, which can be applied by a coating method onto an electrical heater since then the manufacture is particularly simple. Precious metal catalysts such as Pt or Rh as well as transition metal oxide catalysts such as V2O5, CuO, MnO2, can be applied reliably by a coating procedure.
In situ heated oxidation catalysts can be operated efficiently. They can be used especially for the removal of contaminants from exhaust gases by a catalytic oxidation of the toxic contaminants with oxygen remaining in the gas.
This is true for stationary as well as mobile systems. In another embodiment of the invention, th catalyst consists therefore of materials on the basis of precious metals (for example Pt or Rh), of transition metal oxides such as CuO, MnO2, V2O5 or MoO3 or of a member of the perovskite group.
In another embodiment of the invention, the catalyst is arranged in an exhaust gas duct for example, the exhaust pipe of a motor vehicle.
Areas of application for the arrangement according to the invention are:
catalytic oxidation of toxic hydrocarbons such as dioxene, furane, and/or polycyclic aromatic hydrocarbons (PAH) or, respectively, of reaction gases such as CO in the exhaust gases of the respective combustion apparatus.
catalytic oxidation of carbon aerosols of the exhaust gases of internal combustion engines, (for example, Diesel carbon particle catalytic converter).
It is of course, also possible to replace a burner system in a heating apparatus by an arrangement according to the invention, wherein the thermal energy is generated by a catalytic oxidation of the fuels used (for example, natural gas). Because of the discontinuous operation of such apparatus, an appropriate thermal activation of the catalyst is needed before each heating cycle if a catalyst-equipped heating system is employed. The semiconductor carrier material furthermore increases the energy efficiency since it acts as an intermediate storage for the energy generated and releases that energy to its environment in a controlled manner.